Prom weapons have been totally redesigned.

: So I'm wondering, does the player have a flashlight
: to turn on?

No.

There's a segment where the game forces flashlights on, although they look rather weird and aren't precisely locked to your view.

343i games and spotlights don't mix.

: It's the outdoor part of Sanghelios that looks really appealing to me. But it
: seemed like the standout. I got the impression that most of the other
: missions featured comparatively sterile environments.

"Sterile" can have a lot of connotations, although the daytime Sanghelios environment is arguably the best-looking in the game.

Graphically speaking, the rock strata appear far less poly-starved than terrain in other environments and the consistent materials don't stress the texture budget, the stone is a good fit for Halo 5's material model, etc.

Artistically, battlefields are well-landmarked so the 78-degree FoV doesn't feel as limiting as in some other environments (a major complaint I have with Halo 5), and the vistas just plain look good.